Definitions that resort to mathematical equations often make a concept seem abstract, or even impractical.
The heat that flows into or out of a system at constant
pressure is equal to the change in enthalpy.
If heat is given off by a reaction, the reaction is termed exothermic.
If heat is taken up by a reaction, the reaction is termed
endothermic.
Hess' Law:
The enthalpy change for a reaction is regardless of the path by which the reaction occurs.
Acme chemical company develops a method to synthesize methyl methacrylate from acetone. They calculate the enthalpy change after the reaction (in joules). Acme patents their process.
Carnivore Chemical develops a different method for the
synthesis of methyl methacrylate from acetone. They calculate
the enthalpy change after the reaction, in joules, and arrive
at the same number.
Methyl methacrylate can be used to polymerize PMMA, a material
used to make windows such as the one in the medlab in the
movie "Aliens" that Lt. Ripley was trying to break with
a chair. The chair bounced back after each impact, because
PMMA was flexible enough to absorb the shock.
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